Zilpaterol is a β-adrenergic agonist used as a feed additive for cattle at slaughter age that has been shown to increase their average daily gain. U.S. Pat. No. 4,900,735 describes zootechnical compositions of racemic trans zilpaterol and its derivatives can be used to increase the rate of weight gain, improve the feed efficiency and increase carcass leanness in livestock, poultry and fish.
Methods for making zilpaterol are known in the art. For example, in U.S. Pat. No. 4,585,770, Fréchet et al. discuss compounds encompassed by a genus characterized as 6-amino-7-hydroxy-4,5,6,7-tetrahydro-imidazo[4,5,1-jk][1]-benzazepin-2(1H)-one derivatives and pharmaceutically acceptable acid addition salts thereof. The derivatives correspond in structure to the following formula:

Here, R can be various substituents, and the wavy lines indicate that the bonds to the 6-amino and 7-OH groups have the trans configuration. This genus encompasses racemic trans zilpaterol when R is isopropyl.
The methods reported in U.S. Pat. No. 4,585,770 use 4,5-dihydro-imidazo[4,5,1-jk][1]benzazepin-2,6,7(1H)-trione-6-oxime as an intermediate. This compound corresponds to the following structure:

As indicated in U.S. Pat. No. 4,585,770, 4,5-dihydro-imidazo[4,5,1-jk][1]benzazepin-2,6,7(1H)-trione-6-oxime may be formed from starting materials that have been long known in the art. U.S. Pat. No. 4,585,770 illustrates the use of two such starting materials. In both examples, the starting materials are used to form 5,6-dihydro-imidazo[4,5,1-jk][1]benzazepin-2,7-(1H,4H)-dione, which, in turn, may be used to make 4,5-dihydro-imidazo[4,5,1-jk][1]benzazepin-2,6,7(1H)-trione-6-oxime.
In one of the examples in U.S. Pat. No. 4,585,770, the starting material is 1,3-dihydro-1-(1-methylethenyl)-2H-benzimidazol-2-one, which is described in J. Chem. Soc. Perkins, p. 261 (1982):
